A high confinement waveguide for use on electro-optic substrates is highly desirable for its ability to decrease the bend radius of optical waveguides. This would facilitate size reduction of devices, more functionality and greater packing density on electro-optic chips. A further benefit is the creation of hybrid PLC-electro-optic chips.
Due to the small index delta between diffused waveguides, and surrounding electro-optic substrate, such as Ti in lithium niobate, the bend radius for waveguides with acceptable loss is currently rather large. This is a major limiting factor to reducing electro-optic device size. A higher confinement monolithically or hybrid integrated waveguide with a greater index of refraction would enable a smaller bend radius and smaller device features. However, if the higher index material used to make the higher confinement waveguide is electro-optically inactive, the optical power must be transferred adiabatically between the high confinement waveguide and lower confinement electro-optically active waveguide. A structure is needed which can tighten the mode field of the optical signal for passive features, like bends, while still permitting as much transmission within the electro-optic substrate as possible in other portions of the device. Monolithic or hybrid vertical integration of low and high confinement waveguides is also desired as it will lower the total cost of the device by eliminating the need for butt-joint optical transitions between substrates made of different materials, which require precision alignment.
Planar lightwave circuits (PLC) are a well developed passive optical technology. Most common is a silica-on-silicon structure in which waveguides having a core of doped silicon dioxide (SiO2) are deposited on an undoped silicon dioxide cladding layer, lithographically etched, and are subsequently coated with an undoped silicon dioxide upper cladding layer. The doped silica core has a slightly higher optical index of refraction than the cladding. Waveguides have also been made in silicon nitride SiN on a silicon substrate. The core of the silicon nitride waveguide must be much thinner and narrower than the silica waveguide in order to allow only one guided mode to exist, because the SiN index of refraction is likely to be much higher than the doped SiO2, making the index change, Δn, much higher.
A hybrid passive optical waveguide is described in an article by Y. Shani et al, “Integrated optic adiabatic devices on silicon” in IEEE Journal of Quantum Electronics, Vol. 27, No. 3, March 1991, pp 556-566. In that hybrid waveguide 1, as shown in FIG. 1, a stoichiometric SiN strip (Si3N4) is fabricated as an inner core 2 within the doped SiO2 core 4. Most of the light is guided within the Si3N4 strip 2 in this hybrid waveguide 1. FIG. 2 shows an adiabatic taper 3 in the lateral width of Si3N4 2, described by Shani above, that allows the optical power carried in the Si3N4 strip 2 to be transferred into the larger mode doped SiO2 core 4, or vice-versa, without change of mode or loss of optical power. FIG. 3 shows overlapping tapers 5, 7 between conventional silica 8 and SiN 6 waveguides also described by Shani et al. that allow adiabatic transfer of power. The cross section of the overlap region is similar to FIG. 1 for the portion where the doped SiO2 core 4 is wider than the SiN strip 2.
Prior art U.S. Pat. No. 4,737,015 describes an “oxi-nitride” layer on top of lithium niobate that is used to create a stress-induced waveguide. The “oxi-nitride” layer is a blend of SiO2 and SiN. U.S. Pat. Nos. 6,670,210 and 6,864,512 also describe a waveguide containing SiO2 and SiN. It is important to note that the refractive index of the “oxi-nitride” referenced in these patents is not high enough to function as a waveguide core, with lithium niobate as an undercladding substrate. In fact, stoichiometric SiN (Si3N4) has an optical index which is too low to create a waveguide core directly over a lithium niobate substrate. In the prior art SiN is used to form both the core and the cladding by varying the amount of nitrogen to obtain the refractive index difference. Alternatively, SiO2 is used as a cladding layer. However, this provides too much confinement for an electro-optic device.
What is needed for a high confinement waveguide on an electro-optic substrate, is a material having a higher refractive index than the electro-optic substrate that can reduce the mode size of the optical signal. To transfer the optical signal to and from the diffused waveguide, the refractive index of the high confinement waveguide must be at least equal to or higher than the refractive index of the diffused waveguide. The diffused waveguide has an inhomogeneous refractive index with a maximum index at the top center. By contrast the high confinement waveguide has a homogeneous refractive index and this should be higher than an average index of the diffused waveguide. Furthermore, the optical absorption and optical scattering losses must be low. To be practical, the propagation loss in the SiN:Si on lithium niobate should be less than 1 dB/cm.
In order to get a high enough index to create a waveguide core confined by lithium niobate, the SiN must be silicon-rich. Silicon-rich silicon nitride waveguides are described, for example in U.S. Pat. No. 6,470,130, as silicon nitrides having a ratio of greater than 3 silicon atoms to 4 nitrogen atoms per molecule. Silicon nitride compounds having the formula Si3N4 are considered stoichiometric. Silicon nitride compounds with higher silicon content are considered silicon-rich silicon nitrides, written as SiN:Si. The silicon content of silicon nitride is controlled by changing the gas flow parameters and temperature during deposition. As the gas parameters are changed, the index of refraction is affected as well.
Stoichiometric SiN (Si3N4) waveguides are described in Shani, discussed above, and in an article by N. Daldosso, et al., “Comparison among various Si3N4 waveguide geometries grown within a CMOS fabrication pilot line,” IEEE Journal of Lightwave Technology, Vol 22, No 7, July 2004, pp. 1734-1740. Patches of SiN under a silica (SiO2) core have been used to compensate for birefringence, as described by H. H. Yaffe, et al., “Polarization-independent silica-on-silica Mach-Zehnder interferometers,” IEEE journal of Lightwave Technology, Vol 12, No 1, January 1994, pp. 64-67. SiN waveguides have been fabricated which have air as a top cladding and SiO2 as a bottom cladding, described by T. Barwicz, et al., “Fabrication of add-drop filters based on frequency-matched microring resonators,” IEEE Journal of Lightwave Technology, Vol 24, No 5, May 2006, pp 2207-2218. Liquid has also been used as a top cladding with a grating in a Si3N4 core as described in W. C. L. Hopman, et al., “Quasi-one-dimensional photonic crystal as a compact building block for refractometric optical sensors,” IEEE Journal of Selected Topics in Quantum Electronics, Vol 11, No 1, January/February 2005, pp 11-16.
A Si3N4 waveguide integrated with an electro-optically active polymer is described in I. Faderl, et al., “Integration of an electrooptic polymer in an integrated optic circuit on silicon,” IEEE Journal of Lightwave Technology, Vol 13, No 10, October 1995, pp 2020-2026.
However, the prior art does not provide any teaching concerning the creation of high confinement optical waveguides for use on an electro-optic substrate. For the reduction of device size and flexibility of design, such a waveguide structure is highly desirable.
An object of the present invention is to provide a high confinement waveguide for use on an electro-optic substrate and which can be optically coupled substantially adiabatically into a waveguide within the electro-optic substrate.
A further object of the present invention is to provide a high confinement waveguide on the electro-optic substrate having a small bend radius for higher device packing density.
A further object of the present invention is to provide a high confinement optical waveguide adapted to couple light from an electro-optic device into a passive optical device in an integrated hybrid optical device.
A further object of the present invention is to provide an electro-optic device including high confinement waveguides defining small device features and folded features for high packing density.
A further object of the present invention is to provide passive-electro-optic integrated devices including high confinement waveguides providing adiabatic light transfer from the passive to electro-optic device and vice versa.